Two common ways of measuring the moisture content of a material such as tobacco are known as the Kappa system which employs a Kappa moisture meter and the oven method.
The Kappa moisture meter measures the permittivity, or dielectric coefficient of the tobacco. This can, in turn, be correlated with moisture content. The Kappa meter uses a heterodyne circuit to process the signal. In general, the meter measures permittivity at radio frequencies. Radio frequency capacitor type moisture meters have been used by the paper, grain and tobacco industries for years. Although these meters have become more sophisticated through the years, most are still difficult to read and difficult to automatically correct for temperature variation.
The oven moisture tests are of a different nature and do not relate to the radio frequency type of process for relying on permittivity to determine moisture content. They are more difficult and time consuming to employ.
Most moisture meters presently used by the tobacco industry, for example, operate on one of three principles of measurement; dielectric constant, infrared absorption, and microwave absorption. The meter of the present invention measures the dielectric constant. The difference between this meter and others using the same principle of measurement lies in the method of detection. Ordinarily, the change in dielectric constant due to a change in sample moisture content is measured by mixing two radio frequencies (R.F.) signals in an R.F. bridge. The resulting output is the difference between the two. The detection system employed by the circuit of this invention is based on a phase-locked loop. Two oscillators are used in a manner which makes the system considerably more sensitive to changes in sample dielectric constant than previous capacitance type moisture meters.
Phase-locked loops have been used for many years. Originally they made use of vacuum tubes and were very large and complex. Therefore, aside from use as frequency controllers for high power, high frequency transmitters, they were not very practical. In recent years, with the advent of transistors, the size of the phase-locked loop has been drastically reduced. However, initially they were still complex and expensive. As the state of the art progressed from transistors to monolithic integrated circuits (IC), the phase-locked loop became smaller and less complex. Just a few years ago a phase-locked loop was developed containing essentially all of its components on one IC chip. Naturally, price and complexity, as well as physical size, decreased tremendously. Phase-locked loops are now very popular as FM and T.V. detector circuits, local oscillator control in VHF receivers and transmitters, and as frequency synthesizers.
Accordingly, improvements in radio frequency type meters are still in demand and particularly to develop and provide for increased versatility in the meter, reduction in cost and reliability.